A tip that I picked up from a sword collector for preserving polished blades, which are bare metal, is to use common petroleum jelly.
As it was explained to me, cosmoline is basically vaseline with rust inhibitors added.
I use it on blades that are stored in their scabbard. Clean stuff, I even use it on my hunting knives for storage. The nice thing is, a quick wipe with a rag and the stuff comes off ready to slice a piece of sausage without getting that cosmo flavour.
I also give my spare parts a coating for long term storage. I have had some stuff stashed away for close to 20 years, the petroleum jelly has not dried up and there have been zero rust problems.
I am not just pulling this idea out of my 'ear'. Originally, Lee Metford/Enfields were issued with what we call an 'oiler' bottle, but originally it did not contain oil. It had a spoon in the lid. The spoon is for scooping out petroleum jelly. The rifles would be preserved by the soldier with petroleum jelly on the metalwork, and Rangoon oil for lubricating the moving parts.
And yes, with ongoing storage, anything that is preserved, including a gun collection, should be inspected for corrosion at regular intervals. Do the best that you can with oil, grease, dry conditions, but pull each one out at least once a year and give it a look over.